1. Field of the Invention
This invention relates to an improved amplifying circuit for use in an audio amplifier or the like.
2. Discussion of the Prior Art
An illustrative embodiment of a conventional amplifying circuit for use in audio amplifier is shown in FIG. 1 where 1 is an input terminal connected to the base of a NPN transistor 2. 3 is a B+ terminal and 4 is a B- terminal. The resistors 5 and 6 are connected between two terminals 3 and 4 in series, the mid point between resistor 5 and resistor 6 being connected to the base of transistor 2. The emitter of transistor 2 is connected to B- terminal 4 by way of resistor 7 and the collector is connected in common with the collector of a PNP transistor 8 to an output terminal 9. A series circuit comprising a diode 10, a resistor 11 and a resistor 12 is connected between B+ terminal 3 and B- terminal 4 where the mid-point between resistor 11 and resistor 12 is connected to the base of transistor 8. The collector of transistor 8 is connected to B+ terminal 3 by way of resistor 13.
In an amplifying circuit thus constructed, the input signal is amplified by transistor 2 and outputted at output terminal 9 where transistor 8 operates as a constant-current transistor.
The pulse response characteristics (high frequency characteristics) in such an amplifying circuit will now be discussed. When a pulse signal such as shown in FIG. 2(A) is applied to terminal 1, the pulse signal is amplified by transistor 2 as shown in FIG. 2(B) and outputted at output terminal 9. Transistor 2 operates in the on-direction when the pulse signal is rising and in the off-direction when it is falling. It responds quickly in the on-direction; but its response in the off-direction is slow. Therefore, when the output pulse signal waveform shown in FIG. 2(B) is compared to the input pulse signal waveform shown in FIG. 2(A), the rise shows the steep characteristic identical to the input pulse signal waveform, but the fall is different from the steep input pulse waveform and, in fact, is sloped. In other words, in the case of the above-mentioned amplifying circuit, the output pulse signal waveform deviates from the input pulse signal waveform. It has a shortcoming in that the high frequency characteristics are poor.